1. Field of the Invention
The invention relates to the purification of gases in a pressure swing adsorption system. More particularly, it relates to enhancing the adsorption bed utilization in such a system, particularly in high pressure applications.
2. Description of the Prior Art
The pressure swing adsorption (PSA) process provides a highly desirable means for separating and purifying at least one gas component from a feed gas mixture of said gas component and at least one selectively adsorbable component. Adsorption occurs in an adsorbent bed at a higher adsorption pressure, with the selectively adsorbable component thereafter being desorbed by pressure reduction to a lower desorption pressure. The PSA process is commonly employed in multi-bed systems. The Wagner patent, U.S. Pat. No. 3,430,418, discloses a PSA process and system employing at least four adsorption beds arranged for carrying out the PSA processing sequence on a cyclic basis. This sequence includes higher pressure adsorption, cocurrent depressurization to intermediate pressure with release of void space gas from the product end of the bed, countercurrent depressurization or blowdown to a lower desorption pressure, and repressurization to the higher adsorption pressure. Wagner teaches the passing of the released void space gas from one bed directly to another bed initially at its lower desorption pressure. The pressure in the two beds is thereby equalized at an intermediate pressure, after which additional void space gas is released from the one bed as it is depressurized to a lower pressure. The other bed is further repressurized from the intermediate pressure to its higher adsorption pressure at least in part by the countercurrent addition of product effluent to the product end of the bed being repressurized.
In a further development of the art, the Fuderer patent, U.S. Pat. No. 3,986,849, discloses the use of at least seven adsorbent beds, with the feed gas mixture being introduced to the feed end of at least two adsorbent beds, in overlapping identical processing cycles, at all stages of the PSA processing sequence. It is known in the art that advantages can be achieved in particular embodiments by employing a second pressure equalization step in addition to that referred to above. By such a step, a bed undergoing repressurization is further pressure equalized with the void space gas from another bed to a higher intermediate pressure subsequent to the pressure equalization of the bed from its initial desorption pressure to an initial intermediate pressure. In accordance with the Fuderer teaching, each bed, in turn, undergoes three pressure equalization steps prior to final repressurization to the higher adsorption pressure. Fuderer also discloses the carrying out of the three pressure equalization steps in a particular manner to achieve higher product purity by substantially avoiding the impurity profile reversion that can occur upon pressure equalization between the beds, as discussed in the patent. It is also within the contemplation of the art to employ, in some circumstances, a fourth pressure equalization step prior to final repressurization with product effluent.
As noted above, cocurrent depressurization occurs with the flow of void space gas toward the discharge end of the adsorbent bed. During this very useful and important step, product gas is released from the bed at gradually decreasing pressure, and an adsorption front progresses in the bed toward the discharge, or product, end of the bed. The part of the bed in which the adsorption front moves from the start to the end of the cocurrent depressurization step is referred to as the "Front Advance Section" or "FAS." The bed section extending from the inlet, or feed, end of the bed to the stoichiometric point of the adsorption front at the end of the higher pressure adsorption step, in which a feed gas mixture is passed to the inlet end of the bed and product effluent is withdrawn from the discharge end thereof, is referred to as the "Equilibrium Section" or "ES." The FAS is thus a fraction of the total bed, and its size depends on various factors such as the initial and final pressure, molar concentration and bed loading of impurities, adsorbent characteristics and the like. When the concentration of adsorbable impurities in the feed gas is relatively low, e.g. about 5 mol%, and the drop from feed pressure to final cocurrent depressurization pressure is not too great, for example from about 20 bar to about 5 bar pressure, the FAS will be frequently less than 10% of the total bed size. When, on the otherhand, the impurity concentration in the feed is higher, e.g., 30 mol% and the final cocurrent depressurization pressure is lower, as where said pressure drops from about 20 bar to about 3 bar, the FAS may comprise more than 35% of the total bed. In general, it may be observed that the adsorption front advances very little when the bed is depressurized from feed pressure to one half or even to one third of the feed pressure. The adsorption front advances much more as the cocurrent depressurization is continued to lower pressures with the ratio of feed pressure to final cocurrent depressurization pressure increasingly exceeding about 3/1.
Those skilled in the art will appreciate that a large FAS results in several disadvantages affecting the overall PSA process. Thus, the adsorbent bed inventory and the adsorbent vessel size are necessarily higher than if a lower FAS could be employed, adding to the material and apparatus costs associated with the process. In addition, the bed void spaces in the FAS, at the end of the feed-adsorption-product removal step, are filled with product gas at the feed pressure. At a large FAS, the quantity or storage of product gas in the bed is, therefore, large. Furthermore, the product discharge end of the bed is not effectively utilized during the feed-adsorption-product removal step as the ES does not extend as far into the bed as it could if the FAS associated with the process were relatively small. The higher the feed gas mixture pressure employed, the more significantly these disadvantages are found to affect the overall process. High pressure PSA gas separations, e.g., at 20-70 bar feed gas pressures, are of commercial interest with respect to the purification of a variety of gases, such as those produced by (1) partial oxidation of residual oils, ( 2) coal gasification, e.g. coal hydrogenation recycle, and (3) methanol purge gas, hydrotreater and hydrocracker purge gas purifications and the like. Because of the large size of plants required for such commercial separations, as well of the high pressures generally involved, savings on the adsorbent vessel metal requirements and increased product recoveries are desirable to make such separations technically and economically feasible and attractive. Such desired savings, it will be appreciated, necessarily involve a reduction in the FAS of the adsorbent beds employed in the multi-bed, relatively high pressure PSA processing techniques and systems as known in the art.
It is an object of the invention, therefore, to provide a PSA process and system for reducing the FAS and enhancing the utilization of the adsorbent bed employed therein.
It is another object of the invention to provide a process and system for enhancing the recovery of product in PSA processing operations.
It is a further object of the invention to provide a PSA process and system capable of enabling reductions to be made in the metal requirements of the adsorbent beds used for PSA operations.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.